A: The "Eastern Schism" (as it is known in the Roman Catholic Church) or "Great Schism" (as it is known in the Orthodox Church) can be dated to 1054, [ when Cardinal Humbert and two papal legates delivered a bull of excommunication against Patriarch Caerularius of Constantinople (as well as Leo of Achrida and their adherents); other sources give the date of the schism as 1056.
However, things are

not really that simple. There were fractures before 1054 between Eastern and Western Christianity, and there were temporary reconciliations afterwards. For a more thorough coverage of the schism, from both the Orthodox and Roman Catholic viewpoints, see the first two links below.
There was also a Western Schism (referenced in the previous answers listed here) which lasted from 1378 to 1417, but this was a division within the Roman Catholic Church and had nothing to do with Eastern Orthodoxy. See the other link below for more information on this event.
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